Differential thermal analysis (DTA) generally refers to a calorimetric technique for measuring physical properties of a substance by exposing the substance to different temperature regimes. DTA can be employed to measure parameters associated with phase transitions, glass transitions, polymerization/depolymerization, crystallization, softening, sublimation, dehydration, decomposition, oxidation, cure kinetics and so forth. A differential scanning calorimeter (DSC) measures the temperature and heat flow associated with energy-emitting or energy-absorbing (exothermic and endothermic, respectively) material transitions. DSCs are widely used in academic, government and private facilities for research purposes, as well as for quality control and production purposes. Hereinafter, reference will be made to DSC, although it is to be understood to encompass DTA as well.
During DSC testing, the material being analyzed (“sample”) is heated or cooled according to a desired temperature profile. The results, such as differential temperature or heat flow, are measured and analyzed to understand the properties of the sample material. The basic theory of DSC analysis is well understood; the reader is referred to Reading, et al., U.S. Pat. No. 5,224,775 (the '775 patent) and U.S. Pat. No. 3,456,490 (the '490 patent) for details on the theory of operation of exemplary DSC systems. The '775 and '490 patents are herein incorporated by reference in their entirety. An improved DSC device is disclosed in U.S. patent application Ser. No. 09/767,903, entitled “Differential Scanning Calorimeter” which was filed on Jan. 24, 2001, and which is herein incorporated by reference in its entirety.
There are also other well-known thermal analysis techniques, such as Pressure Differential Scanning Calorimetry (PDSC), Pressure Differential Thermal Analysis PDTA), and Differential Photocalorimetry (DPC). The invention described hereafter may also be applied to instrumentation used for these techniques.
Typical DSC instrumentation includes the following basic components: a measurement module, a computer controller and associated software, and a results output device. The measurement module may include an interchangeable DSC cell, a cooling system, and a base cabinet. The DSC cell may also include a heated measurement chamber, which encloses a sensor assembly upon which the material to be analyzed is placed, and a furnace heater, which is used for heating the measurement chamber.
The cooling device may find application when temperature is being increased or decreased. Cooling devices used with DSC instrumentation include various types of heat exchangers, such as gas-cooled heat exchangers, liquid-cooled heat exchangers, and change-of-phase liquid-gas heat exchangers.
In the past, DSC testing was often a laborious, manual process, where a technician would have to load a sample pan with a sample, remove the cover(s) from the DSC cell, insert the loaded sample pan into the DSC measurement chamber, and replace the cover(s). After a test cycle was completed, the cover was removed from the DSC cell, the old sample pan was removed, the new sample pan was inserted, and so forth. If tests were to be conducted on multiple samples (such as might be the case for quality assurance testing in a large-scale manufacturing operation), the overall testing sequence would be very labor-intensive and time-consuming. Additionally, the manual nature of the process made it very likely that the testers would make errors, such as dropping or contaminating samples, misplacing samples, and so forth.
As a result, it was recognized that an apparatus for automatic sample retrieval and placement, an automatic sampler, would be beneficial. Accordingly, various automatic samplers have been developed. Some of these automatic samplers provide for a sample tray to be loaded with samples, which are retrieved and placed into the DSC cell.
However, current automatic samplers suffer significant disadvantages and drawbacks. For example, because some automatic samplers are robotic in nature, calibration becomes a significant issue. A number of factors may alter calibration: replacement of DSC cells; replacement of the sample tray; variations in sample tray size; autosampler component drift and wear; and so forth. Unfortunately, calibration of current automatic samplers is largely manual process. Not only is the calibration difficult and time-consuming, but the result is often suboptimal when performed by less-experienced personnel and/or when performed in a hurry. In fact, users of automatic samplers often avoid performing calibration because of these difficulties. Consequently, the DSC apparatus may begin to provide inaccurate measurements.
Additionally, some prior art automatic samplers perform calibration using a single sensing technique, e.g., an electrical sensor. However, a sensing technique can fail at times, such as when an electrical sensor is impaired by corrosion, oxidation, poor contact, and so forth. As a result, the calibration performed by such prior art automatic samplers can be inoperative or prone to errors.
Moreover, each of the various components in an automatic sampler (including calibration sensors) has its own tolerance and other variations. As a result, every automatic sampler that is produced can be slightly different from the others. Prior art automatic samplers have not taken this difference into account and, as a result, the calibration is suboptimal.
Some prior art autosamplers have employed robotic grippers for gripping sample pans to be placed in the DSC cell. However, prior art grippers have had a number of significant drawbacks. For example, the gripped sample pan is sometimes not centered in the grippers, resulting in difficulties in placement of the sample pan. Prior art grippers sometimes apply uneven pressure to the sample pan, resulting in crimped or damaged sample pans. Pans may stick or adhere to a gripper finger, resulting in misplacement of the sample pan in the DSC cell. Replacement of fingers in the prior art grippers can require removal of a number of parts, making gripper maintenance a difficult task. Finally, some prior art grippers used a sensor, e.g., an electrical sensor, for pan location. However, reliance on a single sensor can lead to pan location failures when this single sensor is not receiving a proper reading.
Accordingly, prior art automatic samplers have not been robust or flexible in terms of the types of equipment they can use. In some cases, only standard DSC cell types or standard pan types (open versus closed, metallic versus ceramic, etc.) can be used. In other cases, only pans with standard dimensions can be used. Sometimes, the sample tray can accept only a certain type of sample pan having certain dimensions. This greatly limits the flexibility of the automatic sampler.